THEORY OF OPERATION
The AD536A embodies an implicit solution of the rms equation
that overcomes the dynamic range as well as other limitations
inherent in a straightforward computation of rms. The actual
computation performed by the AD536A follows the equation
=
rmsV
V
AvgrmsV
IN
2
Figure 6 is a simplified schematic of the AD536A. Note that it is
subdivided into four major sections: absolute value circuit
(active rectifier), squarer/divider, current mirror, and buffer
amplifier. The input voltage (V
IN
), which can be ac or dc, is
converted to a unipolar current (I
1
) by the active rectifiers
(A
1
, A
2
). I
1
drives one input of the squarer/divider, which has
the transfer function
I
4
= I
I
2
/I
3
The output current, I
4
, of the squarer/divider drives the current
mirror through a low-pass filter formed by R1 and the exter-
nally connected capacitor, C
AV
. If the R1 C
AV
time constant is
much greater than the longest period of the input signal, then
I
4
is effectively averaged. The current mirror returns a current
I
3
, which equals Avg[I
4
], back to the squarer/divider to complete
the implicit rms computation. Thus,
I
4
= Avg[I
I
2
/I
4
] = I
I
rms
14
+V
S
–V
S
I
3
I
2
I
1
I
OUT
R
L
V
IN
|V
IN
|R
–1
ABSOLUTE VALUE;
VOLTAGE-CURRENT
CONVERTER
ONE-QUADRANT
SQUARER/DIVIDER
CURRENT MIRROR
Q1
Q2
Q3
Q4
Q5
COM
4 9
dB
OUT
5
BUF
OUT
6
3
8
1
BUF
IN
BUFFER
7
10
0.4mA
FS
A3
NOTES
1. PINOUTS ARE FOR 14-LEAD DIP.
0.2mA
FS
R1
25kΩ
R2
25kΩ
12kΩ
25kΩ
12kΩ
R4
50kΩ
R3
25kΩ
80kΩ
00504-106
A4
A2
A1
Figure 6. Simplified Schematic
The current mirror also produces the output current, I
OUT
, which
equals 2I
4
. I
OUT
can be used directly or can be converted to a
voltage with R2 and buffered by A4 to provide a low impedance
voltage output. The transfer function of the AD536A results in
the following:
V
OUT
= 2R2 × I rms = V
IN
rms
The dB output is derived from the emitter of Q3 because the
voltage at this point is proportional to –log V
IN
. The emitter
follower, Q5, buffers and level shifts this voltage so that the dB
output voltage is zero when the externally supplied emitter
current (I
REF
) to Q5 approximates I
3
.
CONNECTIONS FOR dB OPERATION
The logarithmic (or decibel) output of the AD536A is one of
its most powerful features. The internal circuit computing dB
works accurately over a 60 dB range. The connections for dB
measurements are shown in Figure 7.
Select the 0 dB level by adjusting R1 for the proper 0 dB reference
current (which is set to cancel the log output current from the
squarer/divider at the desired 0 dB point). The external op amp
provides a more convenient scale and allows compensation of
the +0.33%/°C scale factor drift of the dB output pin.
The temperature-compensating resistor, R2, is available online
in several styles from Precision Resistor Company, Inc., (Part
Number AT35 and Part Number ST35). The average temperature
coefficients of R2 and R3 result in the +3300 ppm required to
compensate for the dB output. The linear rms output is available
at Pin 8 on the DIP or Pin 10 on the header device with an output
impedance of 25 kΩ. Some applications require an additional
buffer amplifier if this output is desired.
For dB calibration,
1. Set V
IN
= 1.00 V dc or 1.00 V rms.
2. Adjust R1 for dB output = 0.00 V.
3. Set V
IN
= +0.1 V dc or 0.10 V rms.
4. Adjust R5 for dB output = −2.00 V.
Any other desired 0 dB reference level can be used by setting
V
IN
and adjusting R1 accordingly. Note that adjusting R5 for the
proper gain automatically provides the correct temperature
compensation.
Rev. F | Page 8 of 15
